(Read Exodus 4:18-23)
After God had appeared in the bush, he often spake to Moses. Pharaoh had hardened his own heart against the groans and cries of the oppressed Israelites; and now God, in the way of righteous judgment, hardens his heart against the teaching of the miracles, and the terror of the plagues. But whether Pharaoh will hear, or whether he will forbear, Moses must tell him, Thus saith the Lord. He must demand a discharge for Israel, Let my son go; not only my servant, whom thou hast no right to detain, but my son. It is my son that serves me, and therefore must be spared, must be pleaded for. In case of refusal I will slay thy son, even thy first-born. As men deal with God’s people, let them expect so to be dealt with.
For a more detailed explanation select the link below.
Exodus 4 Bible Commentary – Matthew Henry (concise) (christianity.com)
Exodus 4:22-23
22 You shall tell Pharaoh, ‘Yahweh says, Israel is my son, my firstborn, 23 and I have said to you, “Let my son go, that he may serve me;” and you have refused to let him go. Behold, I will kill your firstborn son.’”